Mixed masked isocyanate composition and its use in powdered paint

ABSTRACT

The invention concerns an isocyanate composition at least partially masked, characterized in that it is masked by at least two agents, one of which at least has a non-carbon carboxylic function. The invention is applicable to organic synthesis.

[0001] The present invention relates to a novel family of maskedisocyanates. The invention relates more particularly to isocyanatesmasked using two masking agents, and to their use in coating techniquesusing powders.

[0002] For reasons associated with environmental protection and safetyat work, efforts are increasingly being made to eliminate the use ofsolvents in coating techniques, and in particular in painting.

[0003] In this context, coating techniques using powders are beingdeveloped more and more.

[0004] Masked isocyanates are beginning to be used, but their use islimited by the fact that there are so few compounds which satisfy thechemical requirements of the powders.

[0005] A first difficulty lies in the problem of finding maskedisocyanates or mixtures of masked isocyanates which remain in powderedform under the usual storage conditions, it being possible for theseconditions to vary considerably from one place to another. This impliesthat these compounds should have a relatively high melting point and/orglass transition temperature (Tg).

[0006] The derivatives which are the subject of the present study do notalways have a sharp melting point and thus, in this case, an apparentmelting point is determined, either using a Koffler block or using amethod of the so-called capillary type (for example the so-called“Büchi” melting point).

[0007] A glass transition temperature can be measured by differentialthermal analysis (DTA) techniques.

[0008] It is also necessary for these compounds to have glass transitiontemperatures and melting points that are low enough for them to be ableto react under the conditions in which the powders are used.

[0009] The compounds derived from crosslinking reactions should, inaddition, not be harmful either to human or animal health or to theenvironment.

[0010] Consequently, one of the aims of the present invention is toprovide a novel family of masked isocyanates which meets the constraintsoutlined above.

[0011] Another aim of the present invention is to provide compositionswhich can be used in powder coating and which contain maskedisocyanates.

[0012] Another aim of the present invention is to provide a process forsynthesizing isocyanates which meet the above constraints.

[0013] These aims and others which will become apparent hereinbelow areachieved by means of a composition whose isocyanate component is formedfrom at least partially masked isocyanate(s), said isocyanate componentbearing at least one non-carbon-based carboxylic function masked with atleast one masking agent.

[0014] The carboxylic function is advantageously in intimate mixturewith the isocyanate component of the composition. To do this, it is evendesirable for said carboxylic function to be grafted onto thecomposition by reaction of a precursor of said composition, of an agentbearing a carboxylic function and of a function which reacts with a freeisocyanate function. In other words, it is desirable for the carboxylicfunction to be grafted onto the isocyanate system via an agent bearingboth a function which is reactive (i.e. which contains a “labile”hydrogen) with the isocyanate function and a carboxylic function (COOHin salt form or, advantageously, in free form).

[0015] This agent bearing a carboxylic function and a function which isreactive (i.e. which contains a so-called labile hydrogen) with theisocyanate function is advantageously a masking agent. This means thatit is released (octanol test) at a temperature of not more than 250° C.,advantageously 200° C. (2 significant figures), preferably 180° C.

[0016] When the carboxylic functions are borne by agents which are notreleased under the firing conditions, in order to conserve thecrosslinking power, it is desirable for the amount of acid borne by theagents which is not released to be not more than ½, advantageously ⅓ ofthe total amount of the isocyanate functions (which are free, masked andlinked to a non-releaseable masking agent).

[0017] The masking agents most commonly used are those mentioned by M.Wicks in his article “blocked isocyanates” [Progress in Organic Coatings(1975), Vol. 3, p. 73].

[0018] In the context of the present invention, the preferred maskingagents are those whose temperature for deblocking (or demasking) fromthe aliphatic isocyanates is at least equal to 90° C., advantageously100° C. (two significant figures), preferably 110° C. (cf. below:octanol test).

[0019] It is desirable for the glass transition temperature (Tg) of themasked isocyanate, when the masking agent under consideration is usedalone to mask the isocyanate targeted by the person skilled in the art,not to be very much below 0° C. (temperature at least equal to about260° K, advantageously about 270° K, preferably about 280° K). This isbecause the carboxylic function, in general, hardly makes it possible toincrease the glass transition temperature (Tg) by more than 30° C. Thisis particularly true for the preferred isocyanates (such as HDT) of thepresent invention, i.e. the isocyanates mainly obtained (i.e.representing at least a half) from monomers (for example HDI) whichcontain at least one, advantageously two, aliphatic functions which areneither secondary nor tertiary nor neopentyl.

[0020] In the present description, the term “about” is used to highlightthe fact that the values following it correspond to mathematicalround-ups, and in particular that when the figure(s) furthest to theright in a number is (are) zeros, these zeros are positional zerosrather than significant figures, except, of course, where otherwisementioned.

[0021] The masking agents can be divided into three main groups:

[0022] those in which the labile hydrogen is borne by a chalcogen,

[0023] those in which the labile hydrogen is borne by a nitrogen,

[0024] those in which the labile hydrogen is borne by carbon.

[0025] Among those in which the labile hydrogen is borne by a chalcogen(preferably light chalcogens, i.e. sulfur and oxygen), the onesespecially used are those in which the chalcogen is an oxygen; among thelatter, mention may be made in particular of:

[0026] products containing an >N—OH linkage, such as, for example,oximes (═N—OH) or hydroxy imides ([—CO—]₂N—OH); and

[0027] phenols (in the broad sense), especially those in which thearomatic ring is electron-poor, such as hydroxypicolines andhydroxybenzoates (cf. for example EP-A 680 984 and WO 98/4608).

[0028] Mention may also be made of the compounds described in patentapplication EP-A 661 278.

[0029] Among those in which the labile hydrogen is formed by a nitrogen,mention may be made in particular of:

[0030] monosubstituted amides, and in particular lactams (the oneusually used is caprolactam),

[0031] imides ([═CO—]₂N—H), especially cyclic amides such assuccinimide,

[0032] unsaturated nitrogenous heterocycles, in particular those whichare 5-membered (advantageously doubly unsaturated), preferablycomprising at least two hetero atoms (preferably nitrogen); among thelatter, mention may be made of diazoles (such as glyoxalines andpyrazoles), triazoles or even tetrazoles.

[0033] mention may also be made of the compounds described in patentapplication EP-A 661 278.

[0034] The masking agents in which the labile hydrogen is borne bycarbon are essentially compounds of malonic nature, i.e. a radical RCH<bearing two electron-withdrawing groups (such as carbonyl [for exampleketone, acid, ester or even acid salt], nitrile, Rf or[perfluoroalkyl]).

[0035] In order to avoid excessively weighting the masked molecules, itis desirable for the masking agents not bearing said non-carbon-basedcarboxylic function to contain not more than 10 carbon atoms,advantageously not more than 7 carbon atoms, preferably not more than 5.

[0036] It is easier and thus preferable for the masking group to bearthe labile hydrogen which reacts with the isocyanate function via anoxygen or a nitrogen to give the —NH—CO—O— sequence (i.e.R—NCO+HO—→R—NH—CO—O—) or the sequence —NH—CO—N< (i.e.R—NCO+HN<→R—NH—CO—N<).

[0037] Thus, it is preferable for said isocyanate component to be acomposition masked with at least two masking agents, at least one ofwhich contains a non-carbon-based carboxylic function, i.e. thecarboxylic function does not bear a carbon-based radical apart from theone which corresponds to the acid, and thus a non-carbon-basedcarboxylic function which does not comprise esters.

[0038] Advantageously, said non-carbon-based carboxylic function ischosen from acids and salts thereof (preferably inorganic salts). It isdesirable for said non-carbon-based carboxylic function to be an acidfunction (—COOH). The free acid form is preferred.

[0039] Thus, it is possible to envisage several groups (preferably 2 forreasons of economy) for masking the isocyanate functions. This diversitycan be achieved by mixing various masked compounds (in general maskedwith a single group) or preferably by co-reaction (successive, byreacting one and then the other masking agent, or simultaneous, bycarrying out the masking action on the mixture of masking agents).

[0040] The presence of a carboxylic acid function (—COOH) and inparticular of a carboxylic acid function directly grafted onto anaromatic ring, advantageously a benzene ring, makes it possible toincrease the melting point of the masked isocyanate. However, in orderto conserve the excellent mechanical properties of the system accordingto the present invention, it is preferable for the amount of acidfunction present (in equivalents) to be not more than about {fraction(9/10)}, advantageously not more than about ⅘, preferably not more than⅔ of the masked isocyanate functions. The beneficial effects of the acidfunctions on the glass transition temperature (Tg) already becomeapparent above about 10%, but it is desirable to achieve a proportion ofat least 20%. The melting point and the glass transition temperature(Tg) increase continually as the acid content increases, up to 100%.However, it is preferable for the content of masking agent bearing anon-carbon-based carboxylic function to be not more than 90%.

[0041] According to one embodiment of the present invention, thesegroups masking an isocyanate component may all be as defined above (cf.formula (I)) or alternatively only some of them may satisfy thesedefinitions. In the latter case, it is preferable for those (i.e. thesum of those) which bear a carbonyl (ester or acid) functionadvantageously to correspond to formula (I) below and to correspond atleast to about 10% (expressed as masked isocyanate function),advantageously about 20%, preferably to ⅓.

[0042] Thus, one of the most advantageous embodiments consists in usingisocyanates which are at least partially masked with a compound bearingan acid function, advantageously with a compound of formula I in which Zis an acid function. In this case, it is advisable that the isocyanatebe masked with a group other than the one bearing the acid function, andthat the acid function of the masking system be between 90 and 10%(expressed as masked isocyanate function). The other masking agent(s)may be either masking agents that are known per se (which meet theconstraints of release temperature or the masking temperature specifiedin the present description), or an ester corresponding to formula I.This latter term of the alternative is one of the preferred embodiments.

[0043] For the synthesis of the compounds according to the presentinvention, reference may be made to the general procedures, inparticular those in patent application No. EP 0 680 984 A, which givegood results for the (optionally partial) masking operation with acompound of formula I.

[0044] Curiously, the presence of a carboxylic group does not pose anyproblem for the masking operation. The masking functions react as if thecarboxylic groups were not there. However, it is preferable to carry outthe masking reaction at temperatures of not more than about 150° C.,advantageously 130° C., preferably 110° C. (if a particularly low degreeof free isocyanate is desired).

[0045] The presence of organic base catalyzes the masking reaction (seebelow) and generally does not exceed 10% (as masked isocyanate functionequivalents), or even 5%.

[0046] It is preferable to carry out the reaction such that, at the endof the additions, the system is close to the isocyanate/masking agentstoichiometry ±10%, 5%, 2%.

[0047] As has already been mentioned according to the present invention,it is preferable for the melting point of the compound or of the mixtureof compounds obtained to show an apparent melting point at least equalto 30° C., preferably 50° C.

[0048] It is also preferable for the glass transition temperature to beat least equal to 20° C., advantageously at least equal to 40° C.

[0049] It is preferable to select the compounds according to the presentinvention such that they react completely with a primary alcohol at 250°C. in less than half an hour.

[0050] It is considered that the reaction is complete if it proceeds to90% or more.

[0051] As has been mentioned above, the isocyanates for which theinvention is most advantageous are those in which the nitrogen atom islinked to an sp³-hybridized carbon, and more particularly aliphaticones, and in particular polymethylene diisocyanates (for example TMDI,tetramethylene diisocyanate, and HMDI [hexamethylenediisocyanate=OCN—(CH₂)₆—NCO]) and the various condensation derivativesthereof (biuret, etc.) and di- and “trimerization” derivatives thereof,(in the field considered, the term “trimer” refers to mixtures derivedfrom the formation of isocyanuric rings from three isocyanate functions;in fact, along with the true trimer, there are heavier products derivedfrom trimerization).

[0052] According to the present invention, it is desirable and sometimesnecessary for the percentage of residual free isocyanate function(expressed as equivalents) attached to a skeleton comprising anexocyclic polymethylene chain (cf. above) to be not more than 5%,advantageously not more than 3%, preferably not more than 1%. Thehighest melting points or glass transition temperatures are obtainedwith percentages not exceeding 0.5%. The contents of aromaticderivatives hydroxylated on the ring are also advantageously low, i.e.not more than 5%, advantageously not more than 3%, preferably not morethan 1%.

[0053] On the other hand, up to a mass content of ⅓, the presence insaid isocyanate of unmasked oligomer (especially trimer) oroligo-condensates of cycloaliphatic monomers such as IPDT or nBDT has avery favorable effect on the glass transition temperature (Tg) and doesnot in any way impede the production of a high-quality coating. Suchcompositions can be readily obtained by mixing the unmasked trimer inthe molten masked compound.

[0054] According to the present invention, the isocyanate isadvantageously masked with at least one masking group bearing at leastone function derived from acidic functions and in particular acid andester functions. The masking can be mixed and can involve severalmasking groups.

[0055] It is desirable that, in the structure of the isocyanate(s), thepart of the skeleton which connects two isocyanate functions shouldcontain at least one polymethylene sequence (CH₂)π in which π representsan integer from 2 to 10, advantageously from 4 to 8. This preferenceaffects the mechanical performance. When there are several sequences,these sequences can be the same or different. In addition, it isdesirable for at least one, preferably all, of these sequences to befreely rotating and thus exocyclic.

[0056] The degree of release is quantified by the octanol test (seebelow).

[0057] According to the present invention, the masked isocyanate, pureor mixed, is obtained from a polyisocyanate, i.e. a product-containingat least two isocyanate functions, advantageously more than two(possibilities of fractional values since it is generally a mixture ofmore or less condensed oligomers), which is itself usually derived froma precondensation or a prepolymerization of unitary diisocyanate(sometimes referred to in the present description as “monomer”).

[0058] In general, the average molecular mass of these prepolymers or ofthese precondensates is not more than 2000 (one significant figure),more commonly not more than 1000 (one significant figure, preferablytwo).

[0059] One notable exception to these values lies in precondensatesbetween polyols with a molecular mass of greater than 2000 and less than15,000. This relates to the number-average molecular mass, {overscore(M)}n, which ranges between 2000 and 15,000 g/mol. The molecular mass isdetermined by gel permeation chromatography (GPC). The gels used in thetechnique are two polystyrene gels (Ultrastyragel® at 10⁴ and 500 Å),THF as solvent and sulfur as standards. However, these prepolymers arenot always the best.

[0060] Thus, among the polyisocyanates used for the invention, mentionmay be made of those of the biuret type and those in which the di- ortrimerization reaction has led to four-, five- or six-membered rings.Among the six-membered rings, mention may be made of the isocyanuricrings derived from a homo- or hetero-trimerization of variousdiisocyanates alone, with other isocyanate(s) [mono-, di- orpolyisocyanate(s)] or with carbon dioxide. In this case, a nitrogen ofthe isocyanuric ring is replaced with an oxygen. Oligomers containingisocyanuric rings are preferred.

[0061] The preferred polyisocyanates are those which contain at leastone aliphatic isocyanate function. In other words, at least one maskedisocyanate function according to the invention is connected to theskeleton via an sp³-type carbon advantageously bearing a hydrogen atom,preferably two. It is desirable for said sp³-type carbon itself to beborne by an sp³-type carbon and advantageously to contain one,preferably two, hydrogen atoms, so as to avoid the isocyanate functionconcerned being in a neopentyl position. In other words, it isrecommended to choose as monomers (which, in general, bear twoisocyanate functions) at least one compound which bears at least onealiphatic function which is neither secondary nor tertiary norneopentyl.

[0062] In the case of the mixture obtained from several (in general two)types of monomer, it is preferable for the monomer(s) which satisfy theabove conditions and/or (advantageously “and”) the condition regardingthe presence of a polymethylene sequence (CH₂)π, to represent at least⅓, advantageously ½, preferably ⅔ of the masked isocyanate functions.Thus, during the study according to the present invention, excellentresults have been obtained with mixtures comprising ⅔ of HMDT(hexamethylene diisocyanate “trimer”) with IPDI or IPDT (IPDI “trimer”),the two being masked according to the invention (the nBDI, norbornanediisocyanate, and its trimer are similar).

[0063] Needless to say, the case in which all of the isocyanates arealiphatic and, what is more, satisfy the above criterion is preferred.

[0064] According to one particularly advantageous embodiment of thepresent invention, an agent corresponding to the general formula ofthose targeted in European patent application EP-A 661 278 can be usedas masking agent bearing said non-carbon-based carboxylic function. Theother agent(s) can also correspond to said general formula, as is thecase for the para-hydroxybenzoic acid/alkyl para-hydroxybenzoate couple.The essence of EP-A 661 278 will be found below.

[0065] The agents leading to the characteristic masking group(s) of theinvention are advantageously chosen from those which are derived fromthe condensation of an aromatic derivative which is hydroxylated on thering and bears a function chosen from nitrile functions, and preferablycarbonyl functions, with an isocyanate. It goes without saying that thecondensation takes place on the phenol function.

[0066] Among the members of this family, those for which it is possibleto determine an apparent melting point should be selected, thismeasurement being carried out at room temperature (20° C.). This meltingpoint should be at least equal to 30° C. (one significant figure),advantageously at least equal to 50° C.

[0067] Among the compounds of formula (I), acids and salts thereof(alkali metal, alkaline-earth metal, ammonium and/or quaternaryphosphonium salts) will be chosen as first masking agent, but the otherscan be chosen as second or third masking agent.

Ar(R)_(n)(Y—Z)_(m)(OH)_(p)  (I)

[0068] in which Ar is an aromatic residue onto which are grafted nsubstituents R, m polar functions Z chosen from nitrites and carbonylgroups, and p hydroxyl functions.

[0069] The values of n, m and p are such that the sum n+m+p is not morethan the number of substitutable members; advantageously, p is not morethan 2 and is preferably equal to 1.

[0070] Advantageously, m is not more than two and is preferably equal to1.

[0071] Advantageously n is not more than 3, it is preferably chosen fromzero, 1 and 2, and is more preferably equal to zero.

[0072] R represents substituents that have no bearing on the maskingreaction and generally correspond to hydrocarbon-based chains, usuallyalkyl chains in the etymological sense of the term, i.e. an alcoholwhose hydroxyl function has been removed.

[0073] Two vicinal substituents R can be joined together to form a ringwhich can be aromatic, for example.

[0074] Z is advantageously chosen from groups containing a carbonylfunction. Among these functions, mention should be made ofalkoxycarbonyl functions (or, in other words, ester functions), theamide function, the ketone function with the preferential condition thatthere are no acidic hydrogens [in other words, the functionadvantageously does not bear hydrogen, or, if it does bear hydrogen, thecorresponding pKa is at least equal to about 20 (one significant figure,preferably 2), more preferably at least equal to about 25] α to thecarbonyl function (ester, ketone or amide]. Thus, the preferred amides(including lactam, or even urea) are advantageously substituted,preferably enough for there to be no hydrogen on the nitrogen of theamide function or such that there are no reactive hydrogens.

[0075] In which Y is chosen from divalent groups, advantageously —O—,—S—, NR′—, —CR′R″— with R′ and R″ chosen from hydrocarbon-basedradicals, advantageously alkyl radicals, of 1 to 6 carbon atoms,advantageously of 1 to 4 carbon atoms, preferably methyl, morepreferably hydrogen, and preferably Y represents a single bond [sic].

[0076] It is preferable for the polar function(s) Z (in general chosenfrom the nitrile function and/or carbonyl functions) not to be vicinalto the group Z, as, for example, in salicylic acid.

[0077] The aromatic residue Ar consists of one or more advantageouslyfused, hetero- or homocyclic rings. It is preferable for Ar not tocontain more than two rings, and preferably not more than one ring.

[0078] The aromatic residue Ar can consist of one or more hetero- orhomocyclic rings, usually homocyclic on account of their ease of access.However, the advantage of 6-membered heterocycles, which have a muchlower release temperature than that of the corresponding homocycles,should be pointed out.

[0079] It is desirable for the total number of carbons in the aromaticderivative hydroxylated on the ring to be not more than 20, preferablynot more than 10 (one significant figure).

[0080] This ring is advantageously 6-membered, the members consisting ofcarbon or nitrogen with the number of substituents required for thevalency of these atoms.

[0081] Among the acids and derivatives, in particular esters, which givethe most satisfactory results, mention should be made of acids graftedonto a benzene ring or onto pyridine rings. Thus, meta-hydroxybenzoicacids, and especially para-hydroxybenzoic acids, and derivatives thereofgive good results.

[0082] According to one particularly advantageous variant of the presentinvention, the hydroxyarylcarboxylic acids, and in particular thehydroxybenzoic acids, can be combined with one (or more) of the usualmasking agents, such as oxime, lactam, pyrazole or triazole, providedthat they satisfy the glass transition temperature (Tg) and meltingpoint constraints.

[0083] The combinations comprising at least one triazole, which isadvantageously unsubstituted, and at least one agent bearing acarboxylic function, chosen in particular from hydroxyarylcarboxylicacids, are particularly advantageous.

[0084] Thus, it is desirable for said masking agent containing acarboxylic function to represent, in equivalents, at least 10% of allthe masking agents, advantageously at least 20% of all the maskingagents, preferably ⅓, but the effects are very marked when theproportion approaches or exceeds 50%.

[0085] However, when the carboxylic functions are borne by agents whichare not released under the curing conditions, in order to conserve thecrosslinking power, it is desirable for the amount of acid borne byagents which are not released to be not more than ½, advantageously ⅓ ofthe total amount of isocyanate functions (free, masked or linked to anunreleaseable masking agent).

[0086] In order to promote the solid form, said masking agent bearing acarboxylic function has, per masking agent, not more than 4,advantageously 2, freely rotating methyl or methylene groups (i.e.groups not engaged in a ring).

[0087] In order to reduce the firing (crosslinking) temperature,urethane-forming catalysts such as DBTDL (dibutyltin dilaurate) can beadded to the formulation, these being added directly into the premix oras a stock mixture.

[0088] The polyols which can be used with the compounds according to thepresent invention are those known to those skilled in the art.

[0089] This is likewise the case for the additives.

[0090] The powdered paints which form the subject of the presentinvention can advantageously be manufactured by fusion-blending thecompounds of the formulation. First, they are preblended in a blenderand then melted, homogenized and dispersed in a single-screw ormulti-screw extruder.

[0091] It is desirable for the mixing, blending and extrusiontemperature not to be more than about 130° C., advantageously not morethan about 110° C., preferably not more than 100° C. (3 significantfigures). It is desirable for the extrusion temperature to be at leastequal to about 60° C., advantageously at least equal to about 70° C.,preferably at least equal to around 80-90° C. The extruded materialobtained is left to cool, it is passed through a mill until a paint ofthe desired particle size is obtained (in general, d₉₀ is not more thanabout 200 micrometers, advantageously not more than 100 micrometers (twosignificant figures) and d₁₀ is at least equal to about 20 micrometers,advantageously at least equal to about 50 micrometers) for the purposeof applying it to metal supports such as steel, aluminum or otheralloys, glass, plastic or wood.

[0092] The ratio between the polyol(s) and the isocyanate(s) is definedby the deblocking stoichiometry. The amount of isocyanates chosen isgenerally the amount stoichiometrically required to react with all ofthe free hydroxyls, with a tolerance of 20%, advantageously of 10%,preferably of 5%. Since it is preferable to have an excess ofisocyanates, slightly unequal ranges are preferred. In other words, theamount of isocyanates to be added is advantageously at least equal toabout 90% and not more than about 120% of the stoichiometric amount;preferably, it is at least equal to 95% and not more than about 110% ofthe stoichiometric amount; the most common, and thus most desirable,range being at least equal to 100% (three significant figures) and notmore than 105% of the stoichiometric amount. When systems with a highproportion of free acid (for example at least ⅔ of the masked isocyanatefunctions, see above) are used, it may be envisaged to increase theratio between isocyanate and hydroxyl function by about 10 to 30 points(%) relative to the above values.

[0093] The catalysts which can be used as catalysts for the maskingreaction do not in any way harm the quality of the coatings. Ifformulations are used which favor a matt or satin coating, theyaccentuate the matt nature of the coating, in particular in the case offormulations whose matt nature is associated with carboxylic functions(see in particular PCT patent application WO 98/04608). When glossyformulations are used, they promote crosslinking.

[0094] Thus, organic bases in which the basic function is borne by anitrogen or phosphorus atom, nitrogen being preferred, can be present inthe composition. It is desirable for the basic atom not to bear anyhydrogen. The basicity is at least equal to that of pyridine rings (forexample pyridine itself, picolines or quinoline). The preferred basesare phosphines or, preferably, tertiary amines. The tertiary amines cancontain from 3 up to about 50 carbon atoms per basic function (it isrecalled that it is more practical for there to be only one permolecule). Heavy amines, especially fatty amines, have a favorableeffect on the smooth aspect of a painted surface.

[0095] Amines also have a favorable effect on the mechanical properties,which indicates a catalysis of the crosslinking in the powder phase.

[0096] Admittedly, it is preferable for the organic bases to be, per se,relatively non-volatile (b.p. at least equal to 80° C., advantageously100° C., preferably 200° C.), but, under the conditions for crosslinkingpowders, the observation of the low volatility is not very restrictingand is, in particular, easily achieved provided that there are free acidfunctions in the binder in an amount at least equal to the amount ofbase (expressed in equivalents). If not, it is possible to envisageusing heavier bases, i.e. bases whose molecular mass is at least equalto 100, advantageously 180, preferably fatty bases, i.e. bases whosemolecular mass is greater than 250. The organic bases can be pure or amixture. The amines can comprise other functions and in particularfunctions corresponding to amino acid functions and cyclic etherfunctions such as N-methylmorpholine, or not. These other functions areadvantageously in a form which does not react with the isocyanatefunctions.

[0097] The amount of amine can be at least equal to about 1% (inequivalents), advantageously 2%, preferably 3% of the masked isocyanatefunctions in the powder. The amines can be used alone or in combinationwith the other components of the powder paints.

[0098] The powder obtained can be applied with an electrostatic gun orby means of a fluid bed. The preferred application of the presentinvention is that carried out with a Corona effect and Corona chargeelectrostatic gun or by friction (triboelectric).

[0099] The substrate onto which the paint is applied, mainly steel oraluminum, may or may not be preheated before the application. Onceapplied, the powder is melted and cured in an oven for 10 minutes to 2hours, at a temperature of between 140 and 220° C. depending on whetheror not the system is catalyzed, in general for 10 to 30 minutes at atemperature ranging from 180° C. to 220° C.

[0100] Given the account hereinabove, a person skilled in the art willadapt the firing recalling that increasing the firing temperature allowsthe time to be reduced, and vice versa.

[0101] The isocyanate compositions and components according to thepresent invention can be used in powder form. They can also be used indissolved form. They are of specific value for use in the form of asuspension, especially when the content of free isocyanate is low (freeisocyanate(s)/total isocyanate ratio (in equivalents) advantageously notmore than 5%, preferably 2%).

[0102] The non-limiting examples below illustrate the invention.

[0103] In all the examples, HDT or HMDT means hexamethylene diisocyanatetrimer known under the trade name Tolonate® HDT and IPDT meansisophorone diisocyanate trimer.

[0104] Octanol Test

[0105] Definitions “Release” (or This is the lowest temperature at“deblocking”) which the agent masking the masked temperature: isocyanateis displaced to an extent of 9/10 (mathematically rounded-up) by aprimary monoalcohol (the primary alcohol is generally octanol). Shelflife: In order to ensure a good shelf life, it is preferable to choosemasked isocyanate functions whose octanol test shows a “release” at 80°C., advantageously at 90° C., of not more than 90%. Reaction progress:The reaction is considered to be complete if it proceeds to more than90%.

[0106] Procedure

[0107] About 5 mmol of protected masked NCO equivalent to be evaluatedare loaded into a Schott-type tube with magnetic stirring.

[0108] 2.5 to 3 ml of 1,2-dichlorobenzene (solvent) and the equivalentof 1-octanol (5 mmol, i.e. 0.61 g, optionally with the catalyst to betested with the masking group) are added.

[0109] The reaction medium is then brought to the test temperature. Itis then heated for 6 h at the test temperature so as to deblock theisocyanate functions and thus make them reactive. Once the reaction iscomplete, the solvent is removed by distillation under vacuum and theresidue is analyzed by NMR, mass spectrum and infrared.

[0110] From these data, the percentage of masked isocyanate functioncondensed with the 1-octanol is evaluated.

[0111] When the boiling point of one of the constituents is higher thanthe temperature which it is desired to test, the process is thenperformed at the corresponding autogenous pressure.

EXAMPLE 1 Of the Synthesis of HDT Masked With a Mixture (80/20 mol permole) of methyl para-hydroxybenzoate and para-hydroxybenzoic Acid

[0112] 136.2 g of Tolonate® HDT product, whose content of isocyanate(NCO) functions is 0.521 mol per 100 g of HDT, are introduced into a 500ml thermostatically-controlled reactor fitted with a mechanical stirrer.86.4 g of methyl para-hydroxybenzoate and 19.6 g of para-hydroxybenzoicacid are successively added. The temperature of the reaction medium[lacuna] brought to 86° C. and 3.7 g of triethylamine are then added.The temperature of the reaction medium is then brought to 100° C. After2 h 45 min at 100° C., the reaction medium is cooled and then ground togive a powder. The content of free isocyanate functions is 1.05% and theTg of the product is about 30° C.

EXAMPLE 2 Of the Synthesis of HDT Masked With a Mixture (65/35 mol permole) of methyl para-hydroxybenzoate and para-hydroxybenzoic Acid

[0113] 200 g of Tolonateo HDT product whose content of isocyanate (NCO)functions is 0.521 mol per 100 g of HDT are introduced into a 500 mlthermostatically-controlled reactor fitted with a mechanical stirrer.113.3 g of methyl para-hydroxybenzoate and 50.4 g of para-hydroxybenzoicacid are successively added. The temperature of the reaction medium isbrought to 26° C. and 3 g of triethylamine are then added. Thetemperature of the reaction medium is then brought to 93° C. After 30minutes at 93° C., the temperature of the reaction medium is brought to130° C. The molten mass is removed, cooled and then ground to give 344 gof powder. The content of free isocyanate functions relative to the HDTis 1.3% and the Tg of the product is about 32° C. Infrared analysis in aKBr disk indicates the presence of peaks for the acid function at 2500cm⁻¹, a very low content, or even an absence, of peaks corresponding tothe acid functions (1650 cm⁻¹ and 1550 cm⁻¹) and the presence ofcarbamate functions.

[0114] The content of potential isocyanate functions is 12.03%.

EXAMPLE 3 Of the Synthesis of HDT Masked With a Mixture (50/50 mol permole) of methyl para-hydroxybenzoate and para-hydroxybenzoic Acid

[0115] The same method as described in Example 2 is applied. 342 g ofwhite powder are thus recovered.

[0116] The content of free NCO, expressed as unmasked HDT, is 0.7%, i.e.0.175% by weight of residual NCO functions. The Tg of the product isabout 35° C.

[0117] The content of potential NCO is 11.95%.

[0118] Infrared analysis shows the characteristic peaks of the expectedproduct.

EXAMPLE 4 Of the Synthesis of a Mixture (85/15% by Weight) of (HDT/IPDT)Masked With a Mixture (65/35 mol per mole) of methylpara-hydroxybenzoate and para-hydroxybenzoic Acid

[0119] The same procedure as in Example 2 is used, except that themixture of the two polyisocyanates HDT and IPDT (119 g and 21 g) isintroduced instead of the HDT, the amounts of masking agent are adjustedand the removal takes place at 160° C. After removal and cooling, theproduct is ground.

[0120] The Tg of the product is about 35° C. and the content of freeisocyanate functions is 0.2% by weight, i.e. 0.82% of unmasked HDT.

[0121] The infrared spectrum shows the peaks characteristic of theexpected product.

EXAMPLE 5 of the synthesis of HDT masked with a mixture (94/6 mol permole) of methyl para-hydroxybenzoate and calcium salt ofpara-hydroxybenzoic Acid

[0122] 141.25 g of Tolonate® HDT product whose NCO content is 0.521 molper 100 g of HDT are introduced into a 500 mlthermostatically-controlled reactor fitted with a mechanical stirrer.111.9 g of methyl para-hydroxybenzoate, 6.07 g of para-hydroxybenzoicacid and 4.4 g of calcium carbonate are successively added. Thetemperature of the reaction medium is brought to 120° C. and maintainedfor 45 minutes. The reaction mixture is cooled to 80° C. and 1.3 g oftriethylamine are then added. The temperature of the reaction medium isthen brought to 95° C. The reaction medium is removed and ground at roomtemperature to give a powder. The content of free NOC [sic] is 0.69% andthe Tg of the product is about 26° C.

EXAMPLES 6 TO 19 Of the Synthesis of HDT Masked With a Mixture of TwoMasking Agents, One of Which Bears a Carboxylic Function

[0123] The procedure is, with the necessary changes (masking agents andpercentage thereof), that of Example 2 when the isocyanate is HDT aloneand that of Example 3 when the isocyanate is an HDT/IPDT mixture. Natureand percentage by mass Masking agents as a of polyisocyanates mol %relative to Tg polyiso- polyiso- the NCO functions in Comp. cyanate 1cyanate Agent 1 Agent 2 ° C. Examples HDT/100  triazole/100  <0    7HDT/100 triazole/90 p-hydroxy- 19 benzoic acid/10  8 HDT/100 triazole/80p-hydroxy- 27 benzoic acid/20  9 HDT/100 triazole/70 p-hydroxy- 29benzoic acid/30 10 HDT/100 triazole/60 p-hydroxy- 30 benzoic acid/40 11HDT/100 triazole/50 p-hydroxy- 38 benzoic acid/50 12 HDT/85  IPDT/15triazole/85 p-hydroxy- 23 benzoic acid/15 13 HDT/85  IPDT/15 triazole/70p-hydroxy- 34 benzoic acid/30 14 HDT/70  IPDT/30 triazole/85 p-hydroxy-38 benzoic acid/15 15 HDT/70  IPDT/30 triazole/70 p-hydroxy- 35 benzoicacid/30 16 HDT/100 triazole/90 para-amino- 15-20 benzoic acid/10 17HDT/100 triazole/80 para-amino- 26 benzoic acid/20 18 HDT/100triazole/90 N salt 15-20 (Nylon salt)*/10 19 HDT/100 triazole/656-amino- 23 caproic acid/35

EXAMPLE 20 Qualitative Tests

[0124] The syntheses were carried out (according to procedure 2) for thefollowing couples which show an increase in the glass transitiontemperature (Tg) relative to the compound containing no masking agentbearing a carboxylic function. carrier of non- isocyanate maskingcarbon-based used agent carboxylic function HDT/100 methyl p-para-hydroxy- hydroxy- phenylacetic benzoate/85 acid/15 HDT/100 methylp- cholesteric hydroxy- acid/15 benzoate/85 HDT/100 methyl p-hydroxyphthalic hydroxy- acid/15 benzoate/85 HDT/100 methyl p-para-hydroxy- hydroxy- benzamide benzoate/85 acid [sic]/15

1. Isocyanate composition which is at least partially masked,characterized in that it comprises at least one non-carbon-basedcarboxylic function and is masked with at least one masking agent. 2.Composition according to claim 1, characterized in that said carboxylicfunction is grafted onto the composition by reaction of a precursor ofsaid composition, of an agent bearing a carboxylic function and of afunction which reacts with a free isocyanate function.
 3. Compositionaccording to claim 2, characterized in that said agent bearing acarboxylic function and a function which reacts with a free isocyanatefunction is a masking agent bearing a carboxylic function. 4.Composition according to claims 1 to 3, characterized in that saidcomposition is a composition masked with at least two masking agents, atleast one of which contains a non-carbon-based carboxylic function. 5.Composition according to claims 1 to 4, characterized in that the ratio(in equivalents) between the carboxylic functions, on the one hand, andthe isocyanate functions (which are masked, free and which have reactedwith any agent bearing carboxylic functions), on the other hand, is atleast equal to 5%, advantageously 10%, preferably 20%.
 6. Compositionaccording to claims 1 to 5, characterized in that the ratio (inequivalents) between the carboxylic functions, on the one hand, and theisocyanate functions (which are masked, free and which have reacted withany agent bearing carboxylic functions), on the other hand, is not morethan about {fraction (9/10)}, advantageously about ⅘, preferably ⅔, morepreferably one third.
 7. Composition according to claims 1 to 6,characterized in that when the carboxylic functions are borne by agentswhich are not released under the firing conditions, in order to conservethe crosslinking power, it is desirable for the amount of acid borne byagents which is not released to be not more than ½, advantageously ⅓ ofthe total amount of the isocyanate functions (which are free, masked andlinked to an unreleaseable masking agent).
 8. Composition according toclaims 1 to 7, characterized in that said non-carbon-based carboxylicfunction is chosen from acids and salts thereof.
 9. Compositionaccording to claims 1 to 8, characterized in that said non-carbon-basedcarboxylic function is an acid function (—COOH).
 10. Compositionaccording to claims 1 to 9, characterized in that said masking agentcontaining a carboxylic function represents, in equivalents, at least10% of all of the masking agents.
 11. Composition according to claims 1to 10, characterized in that said masking agent containing a carboxylicfunction represents, in equivalents, at least 20% of all of the maskingagents.
 12. Composition according to claims 1 to 11, characterized inthat said masking agent containing a carboxylic function contains, permasking agent, not more than 4, advantageously 2, freely rotating methylor methylene groups.
 13. Composition according to claims 1 to 12,characterized in that said masking agent containing a carboxylicfunction corresponds to the formula: Ar(R)_(n)(Y—Z)_(m)(OH)_(p)  (I) inwhich Ar is an aromatic residue to which are grafted n substituents R, mpolar functions Z chosen from the carboxylic acid function and saltsthereof, and p hydroxyl functions; the values of n, m and p are suchthat the sum n+m+p is not more than the number of substitutable members,advantageously p is not more than 2 and it is preferably equal to
 1. 14.Composition according to claims 1 to 13, characterized in that saidcarboxylic function is directly linked to an aromatic ring. 15.Composition according to claims 1 to 14, characterized in that themasking agent not bearing a carboxylic function is chosen from maskingagents which are known per se.
 16. Composition according to claims 1 to15, characterized in that the masking agent not bearing a carboxylicfunction is chosen from oximes, lactams, pyrazoles and triazoles. 17.Composition according to claims 1 to 16, characterized in that saidisocyanate composition is formed from an oligo-condensate or an oligomercontaining at least one masked aliphatic isocyanate function. 18.Composition according to claim 17, characterized in that said maskedaliphatic function is connected to the skeleton via an Sp³-type carbonadvantageously bearing a hydrogen atom, preferably two.
 19. Compositionaccording to claim 18, characterized in that said aliphatic function isneither secondary nor tertiary nor neopentyl.
 20. Composition accordingto claims 1 to 19, characterized in that said isocyanate composition isobtained from an at least partial masking of a mixture obtained byoligomerization or oligocondensation starting with several monomers, atleast one of which contains at least one, advantageously two, aliphaticfunction(s) which is(are) neither secondary nor tertiary nor neopentyl.21. Composition according to claim 20, characterized in that the unit(s)obtained from monomer(s) which contain(s) at least one, advantageouslytwo, aliphatic function(s) which is(are) neither secondary nor tertiarynor neopentyl bear(s) at least ⅓, advantageously ½ and preferably ⅔,masked isocyanate functions.
 22. Composition according to claims 1 to21, characterized in that said isocyanate composition is obtained froman at least partial masking of a mixture obtained by oligomerization oroligocondensation starting with several monomers, at least one of whichcontains a polymethylene chain, the unit(s) obtained from monomer(s)which contain(s) at least one polymethylene chain bearing at least ⅓,advantageously ½ and preferably ⅔, masked isocyanate functions. 23.Composition according to claims 1 to 22, characterized in that themasking agent not bearing a carboxylic function is chosen fromtriazoles.
 24. Composition according to claims 1 to 23, characterized inthat it also comprises an organic base, advantageously a tertiary amine.25. Use, in a powder paint formulation, of the composition according toclaims 1 to
 24. 26. Coating, characterized in that it can be obtainedaccording to claim 25.